The use of an excimer laser as a light source of a reduction projection aligner for exposing patterns of integrating circuits, etc on semi-conductor wafers is attracting public attention. To eliminate the need for correcting the chromatic aberration of reduction projection lenses, the band of a excimer laser beam (referred to hereinbelow simply as oscillated laser beam) is narrowed, and the thus obtained laser beam with a narrow band width is used as the light source for exposure.
The prior art apparatus for controlling oscillating laser with a narrow band width comprises, as shown in FIG. 1, a laser resonator 110 which includes two etalons 101 and 102 located between a laser chamber 100 and a front mirror 104. The arrangement is made such that the wavelength of the laser beam transparent to the respective etalons 101 and 102 can be adjusted by varying the angle of etalons 101, 102, respectively, and the laser beam is repeatedly reflected between a rear mirror 103 and the front mirror 104 thereby exciting it and then oscillating it from the front mirror. The oscillated laser beam is partially reflected by beam splitters 105a, 105b and 105c. In this arrangement, as the means for detecting the wavelength of the oscillated laser beam, a laser wavelength controlling apparatus is used, which is arranged to generate a narrow-band laser beam having a predetermined wavelength by leading the oscillating laser beams reflected by the beam splitters 105b and 105c, respectively, via lenses 106 and 107 into monitor etalons 108 and 109, respectively to thereby detect the wavelength thereof by these two pieces of etalons 108 and 109, respectively, and effecting feed-back control (not shown in the drawing) of the wavelength of the laser beams transparent to the etalons 108 and 109 on the basis of the detected value of wavelength.
However, in case monitor etalons or a diffraction grating type spectroscope are used as the wavelength detector, it was difficult to detect the absolute value of the wavelength of the laser beam, because of fluctuations in the wavelength to be measured due to changes in environmental conditions (such as, for example, the atmospheric temperature and pressure, etc.).
Consequently, in case the wavelength of the oscillating laser beam with a narrow band width was controlled by means of such a wavelength detector, it was impossible to control and stabilize the absolute value of the wavelength thereof with a high accuracy for a long period of time.